Idiopathic juvenile osteoporosis--an analysis of the muscle-bone relationship

Idiopathic Juvenile Osteoporosis (IJO), a disease of unknown etiology, manifests typically by pain, bone deformities and fractures. Due to limits in BMD data interpretation, evaluation of the muscle-bone functional unit has recently been proposed as a means to assess the general competence of the skeleton. The aim of this study was to evaluate skeletal status during the acute phase of IJO and during recovery from the disease in relation to muscles.

The study population comprised 61 IJO children, including 34 girls (mean age: 13.6+/-3.1 years; range: 7-18) and 27 boys (14.3+/-3.3; 5-18 years). DXA total body (TB) and lumbar spine (S) bone mineral content (BMC) and density (BMD) were measured. Lean body mass (LBM) was employed to calculate SBMC/LBM, TBBMC/LBM, body height (BH)/LBM and LBM/body weight (BW) ratios. Previously established references for healthy controls were utilized for the calculation of Z-score values in IJO cases in respect to phase of the disease.

IJO patients had significantly decreased Z-score values for TBBMD, SBMD, SBMC/LBM and TBBMC/LBM ratios but not for the LBM and BH/LBM or LBM/BW ratios. During the acute phase IJO girls had mean Z-scores for TBBMD and SBMD of -2.49+/-0.61 and -3.27+/-1.03, respectively, which were significantly lower than Z-scores during the recovery phase: -0.90+/-0.66, -1.38+/-0.95 (p<0.0001). IJO boys during the acute phase had Z-scores of -2.08+/-0.65 and -2.75+/-1.19 for TBBMD and SBMD, respectively, which were significantly lower than those during the recovery phase (-0.51+/-1.04 and -1.39+/-1.49; p<0.0001). Further, during the acute phase, TBBMC/LBM Z-scores of -2.95+/-1.15 and -2.56+/-1.49 were noted in girls and boys, respectively; the corresponding SBMC/LBM Z-scores were -2.66+/-1.07 and -2.22+/-1.62. During the recovery from IJO, TBBMC/LBM and SBMC/LBM Z-scores of -1.07+/-0.99 and -0.91+/-1.16 and of -1.15+/-1.40 and -0.68+/-1.45 were noted in girls and boys, respectively, and all were significantly higher than those during the acute phase (p<0.0001).

The results of this study indicate that IJO is a bone disorder characterized by an imbalanced muscle-bone relationship and fractures at onset and during the acute phase and by at least a partial recovery without bone pain and new fractures. Implementation of the BH/LBM, TBBMC/LBM and SBMC/LBM ratios to the armamentarium of pediatricians diagnosing bone disorders will provide mechanically meaningful data for diagnostic purposes and, hopefully, for proper therapeutic decisions.